Modulation of gene expression is achieved in part by a conserved regulatory mechanism called combinatorial control, in which permutations among a fixed set of transcription factors at a promoter allow different transcriptional responses. One aspect of combinatorial control is how families of transcription factors that bind the same promoter motif behave differently to allow distinct expression outcomes. We study this issue in a well characterized yeast system that modulates expression of sulfur metabolism genes in response to varying environmental cues. Transcription of these genes depends on one activator called Met4. Since Met4 lacks intrinsic DNA binding ability, Met4 relies on interactions with DNA-binding "cofactors" (transcription factors that lack intrinsic activation ability) to target Met4 to specific promoters. There are two classes of DNA-binding cofactors for Met4 with each class binding a distinct DNA motif. One class consists of the basic helix-loop-helix protein Cbf1;the other class consists of two similar and semi-redundant zinc finger proteins: Met31 and Met32. While we have uncovered that Cbf1 is required for a subset of Met4 target genes to be expressed, all Met4 target gene expression is eliminated upon removal of both Met31 and Met32 [1]. The aim of this proposal is to determine the individual roles of Met31 and Met32 in mediating Met4-activated transcription. Using a genome-wide approach, we will identify targets in terms of promoter binding and transcriptional response and calculate DNA consensus motifs that are specific for Met31 and for Met32. We will also examine how loss of either Met31 or Met32 alters the combinatorial components within the Met4 transcriptional system. When comparing the above data with target promoter composition, we hope to provide more detailed insights into how transcription factor families allow a broad range of transcriptional responses for their targets. The general mechanisms uncovered from this study are likely to be conserved in higher eukaryotes, which contain much larger, more complex, transcription factor families that are intractable to these kinds of analyses. PUBLIC HEALTH RELEVANCE: Transcription factor families, whose members bind the same promoter motif, are found throughout eukaryotic gene regulation systems. These families allow a broad range of transcriptional responses for their targets. By manipulating a well-characterized yeast system, we will examine how a simple two-member transcription factor family orchestrates gene expression of its multiple targets in response to different signaling cues.